During microprocessor verification, different tests used in hardware verification cover different aspects of the design. A coverage measures how much of the design space a test has covered during hardware simulation. Millions of tests, both directed and random are simulated on the hardware design with a goal to measure the coverage of each test. The coverage measured is stored in a file called a coverage database. There can be two types of coverage, structural and functional. Structural coverage measures lines of code or blocks reached by the test. It can also measure flip-flops toggled during simulation. Functional coverage measures if design variables, or their combination have reached values of interest. It also checks if certain conditions were satisfied during the simulation.
A smoke set is used to qualify changes to a design. A smoke set with higher coverage is more likely to detect problems with design changes. This saves debug time down the line. The coverage from different sets can be merged using merge tools and the total coverage seen. Different test databases can also be ranked using rank tools to show which tests have superior coverage. Just selecting a subset of tests with the best coverage, it is possible that there is a significant overlap in coverage amongst those tests. It is also possible that tests with low overall coverage might be excluded and will hurt the overall coverage of the set. Ranking individual tests against each other takes time, money and computation power. Prioritizing test with high coverage has the possibility of missing out low overall coverage tests which provide unique coverage.